Based on the repository's commit message style and the changes in the diff, here's an appropriate commit message:

```
feat: add websocket tests, PoA metrics, marketplace endpoints, and enhanced observability

- Add comprehensive websocket tests for blocks and transactions streams including multi-subscriber and high-volume scenarios
- Extend PoA consensus with per-proposer block metrics and rotation tracking
- Add latest block interval gauge and RPC error spike alerting
- Enhance mock coordinator

apps/blockchain-node/observability/alerts.yml

@@ -41,3 +41,13 @@ groups:
           summary: "No receipts attested in 5 minutes"
           description: |
             Receipt attestations ceased during the last five minutes. Inspect coordinator connectivity.
+
+      - alert: RpcErrorsSpiking
+        expr: increase(rpc_request_failures_total[5m]) > 0
+        for: 5m
+        labels:
+          severity: warning
+        annotations:
+          summary: "RPC error spike detected"
+          description: |
+            RPC request failures have increased during the last five minutes. Investigate rpc_request_failures_total for details.